The present invention relates to a video picture composition system for displaying a plurality of video signals on a display device in a composition manner which is used as both a computer display terminal and a TV display device and, more particularly, to a key signal producing apparatus which generates a key signal for controlling picture composition.
Conventionally, in order to compose a plurality of video signals associated with a plurality of picture elements on a display device, a key signal is used for specifying a position, a size and a shape of each picture element. In response to the key signal, the video signal corresponding to the picture element is gated to form a composite picture. Generally, a key signal producing apparatus includes a key signal generator for generating a key signal for specifying a desired shape, size and position, a memory for storing the key signal for one frame period, a write address generator for generating a write address to store the key signal into the memory, and a read address generator for generating a read address to deliver the key signal from the memory. The key signal generated by the key signal generator is first written in the memory and is then read out. The reason for the inclusion of the memory in the key signal producing apparatus is that the display device of this kind does not always have the same display speed (in terms of horizontal and vertical scanning frequencies) but has various display speeds and, thus, it is necessary to convert a key signal speed associated with the key signal generator to a speed associated with the display device. This speed conversion is realized by matching the reading speed of the memory to the display speed of the display device.
However, the conventional key signal producing apparatus can not produce a key signal to make a picture effect where one picture element is to be moved on the screen in response to a shifting key signal, while another picture element is to be displayed at a certain position without moving in response to another still key signal. This is partly because both key signals would require the use of the same memory; the shifting key signal cannot be outputted, after being generated, by the key signal generator unless first written in the memory, and but the other still key signal will disappear if the succeeding key signal (the shifting key signal) is written therein.